A cellular system is adopted in digital radio communication systems such as portable telephones and car telephones. In the cellular system, a communication area is divided into cells each provided with a base station, and a plurality of mobile stations concurrently communicate with the base station.
This cellular system has been rapidly spread recently, and users thereof have been also rapidly increased, resulting in the important subject that the channel capacity in a cell should be increased.
One of techniques to increase the channel capacity is an adaptive array antenna technique.
In the adaptive array antenna technique, an output from each antenna of an array antenna apparatus comprised of a plurality of antennas is added an amplitude shift and phase shift, and then combined, thereby forming directivities for transmission and reception.
In other words, a weight to be multiplied each antenna output is determined based on a predetermined algorithm, and each weight is varied corresponding to a variation of surrounding environments, thereby controlling the directivities for transmission and reception.
It is possible to attempt to increase the uplink channel capacity by applying the adaptive array technique to uplink channels, and canceling the interference from other cells to improve the reception quality. Concerning this technique, various reports are performed, for example, in academic societies.
Further, the study recently has started to apply the adaptive array technique to downlink channels.
Herein, the application of the adaptive array antenna to downlink channels means that a base station estimates a direction of arrival of the signal from a desired mobile station, and forms the transmission radiation pattern toward an estimated direction to transmit a signal.
The directional transmission with the adaptive array can decrease the transmission power from the base station with the channel quality maintained. Further, the interference to other mobile stations can be reduced. It is thereby possible to attempt to increase the downlink channel capacity.
The above-mentioned directional transmission is generally performed according to the procedures of analyzing a signal received under multipath phenomenon, determining a received level and received quality of the signal for each path, selecting a path such that an estimated propagation environment is the best, and transmitting a signal using the path.
However, it is sometimes difficult to select the optimal path actually. For example, in the case where carrier frequencies are different between transmission and reception, the case occurs that the best path for reception is not the best path for transmission.
Further the case occurs that the best path varies for an instant due to movement of a mobile station and change of propagation environment of radio signal. In such a case, the desirable effect cannot be obtained even if the directional transmission is performed using a path selected at the time of reception as a path for transmission.
The present invention is carried out based on the above-mentioned investigation. An object of the present invention is to enable accurate selection of a path that is optimal for transmission to achieve effective directional transmission.